Kinetic theories have predicted that a partial separation of constituents of a gaseous mixture will occur when the mixture is subjected to a pressure gradient. Industrial processes for separating individual fractions of mixtures on the basis of a pressure gradient are not widespread. In order to obtain sufficient separation between components of a gaseous mixture, relatively steep pressure gradients are required. In the past, large pressure gradients could be achieved in a gaseous mixture using a standard gas centrifuge. Other devices utilizing pressure diffusion sometimes include a separation nozzle, particularly for enrichment of isotopes of uranium.
The typical or standard gas centrifuge includes a tall vertical rotary cylinder fed with the gas mixture to be separated. The cylinder is rotated about its axis at a high angular velocity. The rotation of the cylinder causes the gas mixture to increase its angular rotational velocity so that the lighter components of the mixture move toward the axis and the heavier components of the mixture move toward the wall. Under standard conditions, significant high-purity separation is difficult to achieve unless the rotational velocity is extremely high. A plurality of sequentially ganged or cascaded gas centrifuges are often used to obtain significantly pure components.
Countercurrent gas centrifuges rotate a tall vertical cylinder and also induce an axial convective circulation in order to increase the basic separation effect. The countercurrent flow has been provided using external pumps, by providing an axial temperature gradient or by insertion of a stationary member in the rotating cylinder.
A device known as a separation nozzle uses a concept of a pressure gradient induced in a curved expanding supersonic jet to achieve separation of a gas mixture. The power consumption of separation nozzles is significant relative to the separation achieved. In various prior gas separation centrifuge devices, including countercurrent gas centrifuges and expanding jet or separation nozzle centrifuges, many stages cascaded together have often been required in order to obtain the desired separation.
Another device sometimes previously suggested for gas-gas separation includes a vortex tube or a vortex chamber separator in which a fluidic separation process results from centrifugal forces used for separating or precipitating a denser disperse phase from a lighter continuous flowable phase. Vortex chamber separators have the disadvantage of relatively bad separating efficiency relative to the energy requirement, primarily because of high flow resistance in the vortex chamber and also the use of multi-chamber systems with relatively high volume.
Another centrifuge for separating impurities from gas mixtures, especially for separating sulphur compounds (SO.sub.2) from flue gases, from oil, or from coal-fired furnaces which contain sulphur compounds (SO.sub.2) was disclosed by Wedege in U.S. Pat. No. 4,265,648. A rotor was suggested comprising two separate concentric sets of frustoconical plates with the inlet gas mixture being arranged in the annular space between the two concentric sets. Outlets for the heavy gases were to be at the periphery of the rotatable concentric plate. The SO.sub.2 was to be removed from the periphery and thus cleaned. The remaining gas mixture was to be removed at the central axis. Frustoconical impeller plates in a centrifuge have also been suggested for separating dust particles from suspension in gas as in U.S. Pat. No. 3,234,716.
The separation of dust particles or other solids from flue gases and the separation of heavy gaseous components such as SO.sub.2 having a density at one atmosphere, which is more than two times as dense as the air or the flue gas in which the impurities are carried, have not provided adequate solutions for gas-gas separation where the relative densities of the components of the gas mixture are only slightly different.